The field of the invention is the manufacture of expandable styrene polymers by suspension polymerization and the present invention is particularly concerned with the control of the size and shape of the expandable styrene beads produced.
The state of the art of expandable polystyrene may be ascertained by reference to the Kirk-Othmer, "Encyclopedia of Chemical Technology", 2nd Edition, Vol. 9 (1966), under the section entitled "Foamed Plastics", pages 847-884, particularly pages 852, 853, and 855 where polystyrene is disclosed, and Vol. 19 (1969), under the section entitled "Styrene Plastics", pages 85-134, particularly pages 116-120, where polystyrene foams are disclosed and pages 120, 120 where prior art self-extinguishing polystyrene foams are disclosed and in U.S. Pat. Nos. 4,228,244 and 4,337,319, the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 4,228,244 is incorporated by reference to show the process steps necessary to manufacture molded foam bodies. According to this process, the fine particulate styrene polymers are first heated by means of steam or hot gases to temperatures above their softening points, whereby foaming takes place into discrete particles. This procedure is denoted as pre-foaming. The pre-foamed polystyrenes are then temporarily stored and later further expanded by additional steam heating in a pressure-resistant mold whereby the particles weld into one another to a molded body corresponding to the inside cavity of the mold. This second procedure is denoted as final foaming. The molded object, after final foaming, is cooled inside the mold until the inside temperature drops below the softening point. When the molded object is prematurely removed from the mold, the object deforms. As foam plastics are good insulators, relatively long cooling times are required to cool the mold. The time interval allowing the earliest removal of the molded object without deformation is ordinarily called the "minimum mold dwell time".
U.S. Pat. No. 4,337,319 is incorporated by reference to show the preparation of self-extinguishing, fine particulate, expandable styrene polymers for the manufacture of molded articles.
The state of the art of controlling the size and shape of expandable styrene beads during bead polymerization PG,4 or suspension polymerization may be ascertained by reference to U.S. Pat. Nos. 3,222,343 and 4,036,794; British Pat. No. 1,226,959; French Pat. No. 2,079,991; West German Published Application No. 2,510,937; the Trommsdorf and Meunster article in Schildknecht: Polymer Processes, Vol. 29, pp. 119-120; Houben-Weyl, Methoden der Organischen Chemie, 4th Ed., Vol. XIV, Part 1, pp. 422 and 425; the Winslow and Matrayek article in Industrial and Engineering Chemistry, Vol. 43 (1951), page 1108, and the article by H. Wennig entitled "On the Colloidal Chemistry of Bead Polymerization" as published In Kunststoffe-Plastics, Vol. 5, (1958), pp. 328-340, the disclosures of which are incorporated herein by reference.
Expandable or foamable styrene polymers essentially are produced by the process of bead polymerization or suspension polymerization in the aqueous phase. Present day conventional suspension stabilizers are organic polymers designated as protective colloids. Furthermore, fine particulate powders such as calcium sulfate, barium sulfate, or calcium phosphate may be used to stabilize the suspension droplets. Such stabilizer systems are termed Pickering stabilizers. A listing of commercially available protective colloids can be found for instance in the article by Trommsdorf & Muenster in Schildknecht: Polymer Processes, Vol. 29, pp 119-120.
Substantial significance is attached to the selection of suitable protective colloids for the following reasons:
(1) Setting narrow grain distributions of definite sizes
Foamable bead polymers find applications depending on bead size: coarse beads (2.5 to 0.8 mm) are used in the manufacture of insulating panels, finer fractions (0.8 to 0.4 mm in diameter) are employed in the manufacture of packing materials. It is necessary therefore that the beads always be obtained within the desired range of grain size in adequate amounts, i.e., in high yields.
The proportion of excessively large or small grains should thereby be as small as possible.
(2) Low inner water content of the beads
As regards the conventional suspension polymerization, a certain amount of water is known to be included in the beads. Polymers with a low content of included water evince, in the foamed condition, a uniform foam structure positively affecting the thermal insulation of the foam panels. Accordingly, a minimized amount of included water, called the inner water, is desired.
(3) Spherical bead shape
Defoamed beads are sought when suspension polymerizing takes place with styrene free of expanding agents due to the better workability thereof. However, when producing expandable styrene polymers, the beads should be as spherical as possible.
(4) Adequate suspension stability throughout the entire polymerization cycle
The suspension used in producing expandable styrene polymers is even more unstable than that of styrene polymers free of expanding agents. Accordingly, considering the present day conventional reactor sizes up to 100 m.sup.3, the loss of one batch represents a substantial economic loss. Therefore, phase separation must be assured to be so slow in the case of a malfunction that enough time is available to add a polymerization inhibitor.
Up to the present time, no known suspension system has simultaneously met all of the above requirements. Indeed, many attempts have been made to find a practical way to satisfy all four requirements at the same time. As the disclosed state of the art shows, however, these endeavors were without success.
U.S. Pat. No. 4,036,794 discloses a method using suspension stabilizers which were prepared by the radical polymerization of styrene in the presence of polyvinyl pyrrolidone.
West German Published Application No. 2,510,937 discloses a method wherein the initially low-viscosity system is weakly stabilized by tricalcium phosphate, with post-stabilization a few hours later by means of an aqueous solution of polyvinyl pyrrolidone.
Both methods have the intention of producing styrene polymers with low inner water contents. However, these methods suffer from the drawback that the grain size of the polymer is determined by the point in time at which the organic protective colloid is added.
The accurate determination of the degree of polymerization in heterogeneous mixtures such as are present in suspension polymerization is difficult. Still precise knowledge of the conversion is required for the reproducible setting of the grain spectra because the bead size depends on the particular viscosity of the polymerizing phase at which the protective colloid is added. Furthermore, the polymerizing system remains about two hours in an unsafe operational state, and this feature is especially disadvantageous when using large reactors. Malfunction, for instance agitator failure, especially at the beginning of the polymerization, when most of the styrene is still present, may result in reactor destruction.
British Pat. No. 1,226,959 proposes using two protective colloids, namely polyvinyl alcohol with different degrees of hydroxylation, in order to obtain uniformly large and round beads. As shown in the examples of British Pat. No. 1,226,959, this requires selecting the ratio of styrene to water to be so unfavorable that the method is uneconomical. The method cannot contribute to deliberately controlling the bead grain sizes.
As already mentioned initially, water-insoluble inorganic powders also are used as suspension stabilizers. Calcium phosphates are most commonly used. As a rule, these inorganic compounds are employed with lesser amounts of emulsifiers or surfactants, as disclosed in Houben-Weyl, "Methoden der organischen Chemie", 4th Edition, Vol. XIV, part 1, Macromolecular Substances, page 425. However, compared to the case for organic protective colloids, the application of those systems is restricted, reproducible handling and problem-free suspension polymerization only being possible within a narrow range. The Houben-Weyl reference states on page 422, last paragraph, lines 6 through 8: "Conditions can hardly be stated under which a pulverulent dispersant might be used for a boarder application. The optimal dosage must be precisely observed when combining inorganic compounds with surfactants, batch coagulation being the result both of an excessive and an insufficient dose."
French Pat. No. 2,079,991 discloses how to change the bead shape both by varying the amount of the dispersing agent (protective colloid) and by varying the phase ratio of the aqueous to the organic phases, or also by using a mixture of an organic protective colloid and an inorganic suspension stabilizer. This procedure does not necessarily provide spherical beads or beads with low inner water contents because the dispersing agent is not added to the aqueous phase prior to polymerization. When the dispersing agent is added at the beginning of the polymerization, the grain size cannot be set reproducibly.
Again, similar U.S. Pat. No. 3,222,343 fails to meet the four required conditions listed above.
U.S. Pat. No. 3,222,340 discloses a method for suspension polymerization which is operative in the presence of calcium phosphate acting as the suspension stabilizer. Substantial amounts of a complex forming agent are added to improve the phosphate effectiveness. No teaching of controlling bead size of foamable polystyrene when suspension polymerizing in the presence of organic protective colloids can be inferred from this patent.